1. Field of the Invention
The present invention relates to a dielectric filter in which a conductive film is formed on an internal surface and an external surface of a dielectric block, to a dielectric duplexer, to a communication system using the dielectric filter and the dielectric duplexer, and to a method of producing the dielectric filter.
2. Description of the Related Art
Conventional dielectric filters are disclosed in Japanese Unexamined Patent Application Publication No. 08-316703, Japanese Unexamined Patent Application Publication No. 07-135405, and U.S. Pat. No. 5,162,760.
In Japanese Unexamined Patent Application Publication No. 08-316703, there is disclosed a dielectric filter in which input/output electrodes are formed on a mounting surface and a side surface of a dielectric block, and a gap between an input/output electrode on the mounting surface and an outer conductor is different from a gap between an input/output electrode on the side surface and an outer conductor, thereby decreasing the level of spurious responses.
In Japanese Unexamined Patent Application Publication No. 07-135405, there is disclosed a dielectric filter in which an electrostatic capacitance generated between input/output electrodes and an outer conductor is controlled by changing the width of a gap between the outer conductor and the input/output electrodes, thereby adjusting the phase characteristics.
In U.S. Pat. No. 5,162,760, there is disclosed a dielectric filter in which input/output electrodes are formed with a mesa structure in which a dielectric block is partially cut, on a bottom surface (mounting surface) of the dielectric block, in order to provide isolation between the two input/output electrodes.
However, in Japanese Unexamined Patent Application Publication No. 08-316703 and Japanese Unexamined Patent Application Publication No. 07-135405, the arrangements for controlling the electrostatic capacitance generated between the input/output electrodes and the outer conductor are not disclosed. Therefore, if the dielectric filter is smaller and the distance between the input/output electrodes is shorter, sufficient isolation between the input/output electrodes cannot be assured.
Also, the dielectric filter disclosed in U.S. Pat. No. 5,162,760 has problems in that if the filter is small and the space, in the alignment direction, between a plurality of inner-conductive holes is small, the gap between the two input/output electrodes becomes narrow when the input/output electrodes have a mesa structure and a sufficient isolation cannot be assured between the input and the output, thereby causing unwanted coupling.
In order to address these problems, the present invention provides a dielectric filter and a dielectric duplexer in which the degree of freedom of design is improved and stable characteristics can be obtained. The invention also provides a communication system using the dielectric filter and/or the dielectric duplexer.
According to one aspect of the present invention, there is provided a dielectric filter which has a substantially rectangular-parallelepiped shape including: a dielectric block; an inner-conductor hole in the dielectric block; an inner conductor which coats the inner conductor hole in the dielectric block; an input/output electrode which is coupled to the inner conductor by electrostatic capacitance on the dielectric block; and an outer conductor which is coupled to the inner conductor by electrostatic capacitance on the dielectric block, wherein a gap portion is provided to extend from a bottom surface over a side surface of the dielectric block by cutting a part of the outer conductor and the body of the dielectric block, thereby forming the input/output electrode to be isolated from the outer conductor and allowing filter characteristics to be determined depending on a width or a depth of the gap portion on the side surface.
In the dielectric filter, the electrostatic capacitance between the input/output electrode and the outer conductor can be decreased without reducing an area of the input/output electrode, by providing the gap portion between the input/output electrode and the outer conductor, thereby improving the degree of freedom of design. The electrostatic capacitance generated between the input/output electrode and the outer conductor is decreased, thereby decreasing an effect on the filter characteristics due to the dimensional precision of the input/output electrode. Accordingly, stable filter characteristics can be obtained.
According to another aspect of the dielectric filter, the input/output electrode is provided only on the bottom of the dielectric block and only the gap portion is formed on the side surface of the dielectric block, thereby determining the filter characteristics depending on the width or depth of the gap portion on the side surface. That is, depending on the width or the depth of the gap portion on the side surface of the dielectric block, the electrostatic capacitance generated between the inner conductor and the outer conductor is changed and impedance characteristics of resonators are changed. Thereby, the filter characteristics are determined. The gap portion between the input/output electrode and the outer conductor on the side surface of the dielectric block is enlarged and a changing range of the amount of coupling between the resonator and an external load is increased.
According to other aspects of the dielectric filter, the depth of the gap portion is partly changed, thereby determining the filter characteristics. That is, the degree of change in the electrostatic capacitance generated between the input/output electrode and the outer conductor differs from the degree of change in the electrostatic capacitance generated between the input/output electrode and the inner conductor depending on the changed depth of the gap portion. Accordingly, the depth of the gap portion can be partly changed, thereby determining the filter characteristics.
In other aspects of the dielectric filter, a part of the input/output electrode on the side surface is cut and the width of the gap portion is partly changed, thereby determining the filter characteristics. In this case, the degree of change in the electrostatic capacitance generated between the input/output electrode and the outer conductor differs from the degree of change in the electrostatic capacitance generated between the input/output electrode and the inner conductor depending on a position at which the input/output electrode is partly cut. Accordingly, the position is changed, thereby determining the filter characteristics.
Also, it is possible to almost independently determine the electrostatic capacitance generated between the input/output electrode and the outer conductor and the electrostatic capacitance generated between the input/output electrode and the inner conductor. Thus, a changing range of the filter characteristics can further be increased.
According to another aspect of the present invention, there is provided a dielectric duplexer including a plurality of dielectric filters having the above-described structure wherein three or more of the input/output electrodes are provided on the dielectric block. That is, a plurality of dielectric filters are provided on a single dielectric block and the structure of the input/output electrode has any one of the structures of the above-described dielectric filters, thereby determining the filter characteristics of the dielectric filters.
Further, according to another aspect of the present invention, there is provided a communication system using the dielectric filter and/or the dielectric duplexer. Thus, it is possible to obtain a communication system having excellent high-frequency circuit characteristics by use of a dielectric filter and/or a dielectric duplexer which properly correspond to required characteristics.
Furthermore, according to another aspect of the present invention, there is provided a method of producing a dielectric filter having a dielectric block, an inner-conductor hole in the dielectric block, an inner conductor which coats the inner conductor hole in the dielectric block, an input/output electrode which is coupled to the inner conductor by electrostatic capacitance on the dielectric block, and an outer conductor which is coupled to the inner conductor by electrostatic capacitance on the dielectric block, which includes the steps of: forming the input/output electrode to be isolated from the outer conductor by providing a gap portion extending from a bottom surface and onto a side surface of the dielectric block by cutting a part of the outer conductor and the body of the dielectric block; and determining filter characteristics depending on a width or a depth of the gap portion on the side surface.
Also, the method of producing the dielectric filter includes the steps of: providing the input/output electrode only on the bottom of the dielectric block; forming only the gap portion on the side surface of the dielectric block; and determining the filter characteristics depending on the width or the depth of the gap portion on the side surface.
Further, the method of producing the dielectric filter includes the step of partly changing the depth of the gap portion, thereby determining the filter characteristics.
Furthermore, the method of producing the dielectric filter includes the steps of: cutting a part of the input/output electrode on the side surface; and partly changing the width of the gap portion, thereby determining the filter characteristics.
Moreover, the method of producing the dielectric filter includes the steps of: forming a conductive film over the entire external surface of the dielectric block; and thereafter, providing the gap portion, thereby isolating the outer conductor and the input/output electrode.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings, in which like references denote like elements and parts.